The present invention relates to a two-stage pneumatic servo valve assembly for controlling an output motor. The two-stage servo valve of the present invention has utility in a broad variety of applications, such as controlling vane positions on compressors, exhaust nozzles, and various aircraft function systems.
Two-stage servo valve systems have been used to provide fluid, pneumatic or hydraulic, to an output device such as a piston, wherein piston movement controls a load, such as a motor. Such systems provide for an input mechanism to control a first-stage valve which enables fluid under pressure to be provided to, and control, a second-stage valve. The second-stage valve controls fluid supply to the piston or other similar output device.
One such two-stage servo valve system is disclosed in U.S. Pat. No. 3,745,883. The system of this patent has applications in both pneumatic and hydraulic systems, and includes a second-stage spool valve having springs disposed in chambers at opposite ends of the spool valve to urge the spool valve into a null position. Fluid under pressure from the first stage is directed into the chambers that house the springs and is applied directly to the ends of the spool. The fluid pressure applied to the ends of the spool would appear to be a constant pressure. The second-stage valve controls a piston type of actuator.
U.S. Pat. No. 3,402,737 also discloses a pneumatic servo valve system using floating discs. Such system is both complex and costly to design and manufacture.
Two-stage hydraulic servo valves, such as shown in U.S. Pat. Nos. 3,555,970 and 3,949,645, are also known in the art, but have limited applicability to pneumatic systems. Due to the compressibility of pneumatic fluid, pneumatic systems are much more difficult to design for operation in a stable, responsive manner.